Aphid Repellents

In order to test plant odour components and essential oils as aphid repellents the open Y-track olfactometer (Visser & Piron, 1998) was modified into a T-bar olfactometer as shown in the drawings and the movie frame below. Circular filter papers were placed on both ends of the horizontal part of the T-bar: the filter paper on one side contained the solvent and the paper on the other side the solvent with the odour compound under study. The T-bar was positioned in a black box and was illuminated from above. An individual aphid was gently placed on the base of the T-bar and climbed the stem being attracted to the light. All experiments were conducted in still air, which allowed an odour gradient between the two sides of the T. Arriving at the junction of the T-bar the aphid walked towards the solvent side or the odour side (first choice). On approaching the filter paper with the odour aphids frequently stopped walking, turned around and moved to the control side. As soon as an aphid touched the filter paper the (second) choice was considered to be final. At low light levels most winged aphids just walked and did not fly away. With this simple setup we tested an array of plant odour compounds and essential oils in a short time and we found quite a number of potent aphid repellents.

Movie of Myzus persicae winged aphids on T-bar olfactometer. The left filter paper contains the solvent paraffin oil and the right paper a 10% solution of (-)-(R)-carvone.

T-bar with circular filter papers, Whatman no 1, positioned 1.5 cm from the junction of the T. The T-bar is constructed from a thin brass bar (diameter 1.6 mm), placed vertically in a cork covered by Parafilm M. In the centre of the filter papers paraffin oil solutions were applied: on one side just 10 µl of paraffin oil, the control side, and on the other side 10 µl of a solution of an odour compound (10% or 1% v/v) or essential oil (10% v/v) in paraffin oil.T-bar olfactometer setup: T-bar positioned on a support in a box painted black on the inside (60 x 60 cm, deep 55 cm). The T-bar is illuminated from above. Light intensity at the junction of the T was 80 Lux and at the base of the stem 30 Lux. A black curtain covered most of the open front side of the black box standing in a dark room.

Further down on this page the results of experiments with the peach-potato aphid, Myzus persicae, are shown. We used winged Myzus persicae aphids (alate virginoparae, M3 clone). These aphids were reared on Chinese cabbage at long-day conditions (LD16:8) at 20 oC. Prior to the experiments aphids were starved for 4 to 5 hours. In each test 25 individual aphids were allowed to choose between the solvent paraffin oil and an odour compound or essential oil diluted in paraffin oil. After 5 aphids have been tested the T-bar was rotated 180o in order to prevent any positional bias. The compounds and essential oils tested were all from reliable sources and very pure (mean purity of compounds is 98%). Underneath the test results are shown of odour compounds dissolved in paraffin oil at 10% (v/v).

The percentage of aphids avoiding odour is the percentage of winged Myzus persicae aphids making contact with the control paper (just solvent) on the T-bar. All odour compounds were dissolved in paraffin oil at 10% (v/v). Red bars indicate significant differences between control and odour side (2-tailed P < 0.05, binomial distribution), so these odour compounds diluted at 10% (v/v) repelled aphids.

Quite a number of odour compounds tested repelled Myzus persicae aphids. A selection of these compounds was further tested at a lower concentration, diluted at 1% (v/v) in paraffin oil (see below). Furthermore, a range of plant essential oils was tested as a solution of 10% (v/v) in paraffin oil (see further below).

The percentage of aphids avoiding odour is the percentage of winged Myzus persicae aphids making contact with the control paper (just solvent) on the T-bar. All odour compounds were dissolved in paraffin oil at 1% (v/v). Red bars indicate significant differences between control and odour side (2-tailed P < 0.05, binomial distribution), so carvacrol, (-)-carveol and (-)-(R)-carvone diluted at 1% (v/v) repelled Myzus aphids.

The percentage of aphids avoiding odour is the percentage of winged Myzus persicae aphids making contact with the control paper (just solvent) on the T-bar. All essential oils were dissolved in paraffin oil at 10% (v/v). Red bars indicate significant differences between control and odour side (2-tailed P < 0.05, binomial distribution), so all listed essential oils diluted at 10% (v/v) repelled Myzus aphids.

All essential oils tested at 10% (v/v) repelled Myzus persicae aphids. The compounds carvacrol, (-)-carveol and (-)-(R)-carvone even repelled aphids at 1% dilution. Besides Myzus persicae we tested a number of other aphid species like Aphis fabae, Brevicoryne brassicae, Aphis gossypii, Myzus nicotiana and Macrosiphum euphorbiae. Since most of these experimental series are incomplete they are not presented. Below it is shown that particular odour compounds like (-)-(R)-carvone repelled several aphid species.

The percentage of aphids avoiding odour is the percentage of winged aphids making contact with the control paper (just solvent) on the T-bar. (-)-(R)-carvone dissolved in paraffin oil at 10% (v/v) and 1% (v/v) repelled all three aphid species (2-tailed P < 0.05, binomial distribution).

In the T-bar bioassay individual aphids are allowed to choose twice. After an aphid climbs the stem she arrives at the junction, the horizontal part of the T, and chooses to move towards the right or left (first choice). The aphid then continues to walk to one of the filter papers, or stops, turns around and moves in the opposite direction. The aphid makes a second choice on touching the filter paper on one side. While the first choice results are more or less the same as in any olfactometer test, the second choice outcomes are very convincing. The second choice makes the T-bar bioassay an extremely powerful test. This is illustrated in the figure below.

Analysis of T-bar bioassay: the first choice of aphids when arriving at the junction of the T, and the second choice of aphids, touching one of the filter papers. Data derived from all experiments with winged Myzus persicae in tests with odour compounds (bars at the left, n=900 aphids) and essential oils (bars at the right, n=275 aphids).